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Bad miss & erratic idle can be the T.P.S. had the same problem on an 89 S10 & 91 Sonoma. As for black smoke sounds like oil in cylinders. If the truck has REALLY bad blow by & REALLY worn P.C.V. Valve that can make it smoke too. If not then probably the piston rings with that many miles.

Below
is a listing of vacuum diagrams for most of the engine and emissions
package combinations covered by this manual. Because vacuum circuits
will vary based on various engine and vehicle options, always refer
first to the vehicle emission control information label, if present.
Should the label be missing, or should vehicle be equipped with a
different engine from the car's original equipment, refer to the
diagrams below for the same or similar configuration.Check the link autozone.com (free register), select the year ('91 sample), Brand, Model, Sub-Model, Engine Size and make the next root...1991 GMC TRUCK S15 4WD 4.3L TBI 6CYL REPAIR GUIDE >> EMISSION CONTROLS >> VACUUM DIAGRAMS

yes there is, if this is a tbi they also had two barrell cars this year. if tbi, it is located along the frame rail about where the cab and bed split. I would sugest that you go ahead and pick up a filter at your local parts supplier first so you know what to look for. It will be a can abt 2in in dia and abt 4 in long